diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index e963f49..d4aed67 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -12,6 +12,7 @@
 
 ## Tools
 OPENSSL ?= openssl
+MBEDTLS_CERT_WRITE ?= $(PWD)/../../programs/x509/cert_write
 
 ## Build the generated test data. Note that since the final outputs
 ## are committed to the repository, this target should do nothing on a
@@ -30,6 +31,7 @@
 #### Generate certificates from existing keys
 ################################################################
 
+test_ca_crt = test-ca.crt
 test_ca_key_file_rsa = test-ca.key
 test_ca_pwd_rsa = PolarSSLTest
 test_ca_config_file = test-ca.opensslconf
@@ -64,6 +66,8 @@
 	$(OPENSSL) x509 -req -extfile $(cli_crt_extensions_file) -extensions cli-rsa -CA test-ca-sha256.crt -CAkey $(test_ca_key_file_rsa) -passin "pass:$(test_ca_pwd_rsa)" -set_serial 4 -days 3653 -sha256 -in server2-rsa.csr -out $@
 all_final += server2-sha256.crt
 
+
+
 ################################################################
 #### Generate various RSA keys
 ################################################################
@@ -309,6 +313,85 @@
 ### Generate all RSA keys
 keys_rsa_all: keys_rsa_unenc keys_rsa_enc_basic keys_rsa_enc_pkcs8_v1 keys_rsa_enc_pkcs8_v2
 
+
+
+################################################################
+### Generate certificates for CRT write check tests
+################################################################
+
+### The test files use the Mbed TLS generated certificates server1*.crt,
+### but for comparison with OpenSSL also rules for OpenSSL-generated
+### certificates server1*.crt.openssl are offered.
+###
+### Known differences:
+### * OpenSSL encodes trailing zero-bits in bit-strings occurring in X.509 extension
+###   as unused bits, while Mbed TLS doesn't.
+
+test_ca_server1_db = test-ca.server1.db
+test_ca_server1_serial = test-ca.server1.serial
+test_ca_server1_config_file = test-ca.server1.opensslconf
+
+server1.csr: server1.key server1_csr.opensslconf
+	$(OPENSSL) req -keyform PEM -key server1.key -config server1_csr.opensslconf -out $@ -new
+all_final += server1.csr
+
+server1.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 version=3 output_file=$@
+server1.noauthid.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) not_before=20110212144406 not_after=20210212144406 md=SHA1 authority_identifier=0 version=3 output_file=$@
+server1.der: server1.crt
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+all_final += server1.crt server1.noauthid.crt server1.der
+
+server1.key_usage.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 key_usage=digital_signature,non_repudiation,key_encipherment version=3 output_file=$@
+server1.key_usage_noauthid.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 key_usage=digital_signature,non_repudiation,key_encipherment authority_identifier=0 version=3 output_file=$@
+server1.key_usage.der: server1.key_usage.crt
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+all_final += server1.key_usage.crt server1.key_usage_noauthid.crt server1.key_usage.der
+
+server1.cert_type.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 ns_cert_type=ssl_server version=3 output_file=$@
+server1.cert_type_noauthid.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 ns_cert_type=ssl_server authority_identifier=0 version=3 output_file=$@
+server1.cert_type.der: server1.cert_type.crt
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+all_final += server1.cert_type.crt server1.cert_type_noauthid.crt server1.cert_type.der
+
+server1.v1.crt: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa)
+	$(MBEDTLS_CERT_WRITE) request_file=server1.csr issuer_crt=$(test_ca_crt) issuer_key=$(test_ca_key_file_rsa) issuer_pwd=$(test_ca_pwd_rsa) version=1 not_before=20110212144406 not_after=20210212144406 md=SHA1 version=1 output_file=$@
+server1.v1.der: server1.v1.crt
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+all_final += server1.v1.crt server1.v1.der
+
+# OpenSSL-generated certificates for comparison
+# Also provide certificates in DER format to allow
+# direct binary comparison using e.g. dumpasn1
+server1.crt.openssl server1.key_usage.crt.openssl server1.cert_type.crt.openssl: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa) $(test_ca_server1_config_file)
+	echo "01" > $(test_ca_server1_serial)
+	rm -f $(test_ca_server1_db)
+	touch $(test_ca_server1_db)
+	$(OPENSSL) ca -batch -passin "pass:$(test_ca_pwd_rsa)" -config $(test_ca_server1_config_file) -in server1.csr -extensions v3_ext -extfile $@.v3_ext -out $@
+server1.der.openssl: server1.crt.openssl
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+server1.key_usage.der.openssl: server1.key_usage.crt.openssl
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+server1.cert_type.der.openssl: server1.cert_type.crt.openssl
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+
+server1.v1.crt.openssl: server1.key server1.csr $(test_ca_crt) $(test_ca_key_file_rsa) $(test_ca_server1_config_file)
+	echo "01" > $(test_ca_server1_serial)
+	rm -f $(test_ca_server1_db)
+	touch $(test_ca_server1_db)
+	$(OPENSSL) ca -batch -passin "pass:$(test_ca_pwd_rsa)" -config $(test_ca_server1_config_file) -in server1.csr -out $@
+server1.v1.der.openssl: server1.v1.crt.openssl
+	$(OPENSSL) x509 -inform PEM -in $< -outform DER -out $@
+
+server1_all: server1.csr server1.crt server1.noauthid.crt server1.crt.openssl server1.v1.crt server1.v1.crt.openssl server1.key_usage.crt server1.key_usage_noauthid.crt server1.key_usage.crt.openssl server1.cert_type.crt server1.cert_type_noauthid.crt server1.cert_type.crt.openssl server1.der server1.der.openssl server1.v1.der server1.v1.der.openssl server1.key_usage.der server1.key_usage.der.openssl server1.cert_type.der server1.cert_type.der.openssl
+
+
+
 ################################################################
 #### Meta targets
 ################################################################
@@ -316,10 +399,15 @@
 all_final: $(all_final)
 all: $(all_intermediate) $(all_final)
 
-.PHONY: default all_final all keys_rsa_unenc keys_rsa_enc_basic keys_rsa_enc_pkcs8_v1 keys_rsa_enc_pkcs8_v2 keys_rsa_all \
-	keys_rsa_enc_basic_1024 keys_rsa_enc_basic_2048 keys_rsa_enc_basic_4096 keys_rsa_enc_pkcs8_v1_1024		 \
-	keys_rsa_enc_pkcs8_v1_2048 keys_rsa_enc_pkcs8_v1_4096 keys_rsa_enc_pkcs8_v2_1024				 \
-	keys_rsa_enc_pkcs8_v2_2048 keys_rsa_enc_pkcs8_v2_4096
+.PHONY: default all_final all
+.PHONY: keys_rsa_all
+.PHONY: keys_rsa_unenc keys_rsa_enc_basic
+.PHONY: keys_rsa_enc_pkcs8_v1 keys_rsa_enc_pkcs8_v2
+.PHONY: keys_rsa_enc_basic_1024 keys_rsa_enc_basic_2048 keys_rsa_enc_basic_4096
+.PHONY: keys_rsa_enc_pkcs8_v1_1024 keys_rsa_enc_pkcs8_v2_1024
+.PHONY: keys_rsa_enc_pkcs8_v1_2048 keys_rsa_enc_pkcs8_v2_2048
+.PHONY: keys_rsa_enc_pkcs8_v1_4096 keys_rsa_enc_pkcs8_v2_4096
+.PHONY: server1_all
 
 # These files should not be committed to the repository.
 list_intermediate: